The thresholds that define it

A haemoglobin result on its own tells you very little until you know which threshold it is being read against, and the threshold moves during pregnancy because your blood volume expands faster than your red cell mass does.

NICE Clinical Knowledge Summaries set them out plainly. Anaemia is defined as a haemoglobin level two standard deviations below the normal for age and sex. "In pregnant women - Hb below 110 g/L throughout pregnancy. An Hb level of 110 g/L or more appears adequate in the first trimester, and a level of 105 g/L appears adequate in the second and third trimesters. Postpartum - below 100 g/L." For comparison, the non-pregnant female threshold is 120 g/L.

That is why a haemoglobin of 107 g/L can be reported as normal at 30 weeks and abnormal at 9 weeks. It is not an inconsistency; it is the same rule applied at two points on a moving baseline.

Anaemia is common enough that these numbers get used a lot. WHO estimates that 37% of pregnant women aged 15 to 49 worldwide were affected by anaemia in 2019. Tommy's reports that iron-deficiency anaemia "affects around 4 in 10 pregnant people in the UK".

When you are tested

You do not have to ask for this. Tommy's describes the standard UK schedule: "Your midwife will offer you a blood test at your booking appointment and again when you are 28 weeks pregnant. You will be offered an extra blood test at 20-24 weeks if you are pregnant with more than 1 baby."

You also do not have to wait for those appointments. If you develop symptoms in between, contact your midwife or GP. The symptoms Tommy's lists are non-specific and easy to write off as ordinary pregnancy: "tiredness and lack of energy; shortness of breath; a noticeably fast-beating, fluttering or pounding heart (heart palpitations); paler than normal skin (Black or Brown skin may look grayish or yellowish-white)."

Why pregnancy causes it

Iron deficiency is the commonest cause, and the mechanism is mostly arithmetic. Tommy's puts the demand plainly: "In the second half of pregnancy you need twice as much iron compared to when you are not pregnant." Add the other routes to deficiency - too little dietary iron, poor absorption (coeliac disease is the classic example), and blood loss - and it becomes clear why the third trimester is when most cases show up.

Iron is not the only cause. Vitamin B12 and folate deficiency also produce anaemia, and they need different treatment, which is why a low haemoglobin usually prompts further blood tests rather than an immediate iron prescription. The NHS notes that in adults generally, iron deficiency anaemia is investigated for a source of blood loss where one is not obvious.

What being anaemic changes

Tommy's is careful here, and so is the evidence: "Most people with iron deficiency anaemia in pregnancy go on to have a healthy pregnancy and baby." Untreated, it is associated with a higher risk of low birthweight, premature labour and birth, stillbirth, iron deficiency in the baby's first three months, fatigue in the months after birth, infection and postnatal depression.

Anaemia also matters for how well you tolerate blood loss at birth. The RCOG's advice for anyone with a low-lying placenta is a good illustration of the principle: "You should try to avoid becoming anaemic during pregnancy by having a healthy diet and by taking iron supplements if recommended by your healthcare team." The point of correcting it before term is to give you a buffer.

Treatment, and how to make it work

Oral iron is the first-line treatment. The practical detail is what determines whether it works, and most of it is about absorption rather than dose.

Things that help

Vitamin C with the tablet. Tommy's suggests a glass of fruit or vegetable juice, or a smoothie, with a meal.
Iron-rich food alongside the tablet, not instead of it. Red meat, canned sardines and tuna, poultry, pulses and legumes, dark green leafy vegetables, eggs, dried apricots and raisins, tofu, nuts and seeds, wholemeal and (in the UK, fortified) white bread, and fortified breakfast cereals.

Things that block it

Tea and coffee with or soon after meals, including decaffeinated. They contain tannins, which stop iron being absorbed.
Large amounts of milk and dairy at the same time.

Constipation is the usual reason people stop taking iron. It is worth raising early rather than abandoning treatment, because the alternative preparations and dosing schedules are a conversation your midwife or GP can have with you.

When oral iron is not enough

Your haemoglobin should be rechecked after starting treatment, and a lack of response is information rather than failure. It usually means one of four things: the tablets are not being absorbed, they are not being tolerated and so not being taken, there is ongoing blood loss, or the anaemia is not iron deficiency at all. Each of those has a different answer, which is why the response to a poor recheck is investigation rather than simply a higher dose.

Intravenous iron is used in pregnancy where oral iron has failed or cannot be tolerated, and is generally avoided in the first trimester. That decision sits with an obstetric or haematology team. The British Society for Haematology publishes the UK guideline that services follow, "UK guidelines on the management of iron deficiency in pregnancy", which is the document to name if you want to ask what protocol your unit uses.

After the birth

The postpartum threshold is lower - below 100 g/L - because blood loss at birth is expected and your blood volume is resettling. Anaemia after birth is worth treating rather than waiting out. It overlaps almost completely with the ordinary exhaustion of new parenthood, which is exactly why it gets missed.

If you were anaemic in pregnancy, or lost more blood than usual at birth, ask whether your haemoglobin has been rechecked and whether you should continue iron. Tommy's lists fatigue in the months after birth, infection and postnatal depression among the consequences of untreated iron deficiency anaemia. A blood test is a cheap way to rule one contributor out.

Anaemia in Pregnancy